1. Field of the Invention
The present invention relates to a variable-shape mirror that permits the shape of a mirror surface thereof to be varied, and more particularly relates to a bonding structure between a piezoelectric element and other components constituting the variable-shape mirror. The present invention also relates to an optical pickup device provided with a variable-shape mirror having such a bonding structure.
2. Description of Related Art
When information is read from or written to an optical disc such as a CD (compact disc) or DVD (digital versatile disc) by the use of an optical pickup device, the relationship between the optical axis of the optical pickup device and the disc surface should ideally be perpendicular. In reality, however, when the disc is rotating, their relationship does not always remain perpendicular. As a result, with an optical disc such as a CD or DVD, when its disc surface becomes inclined relative to the optical axis, the optical path of laser light is so bent as to produce coma aberration.
When coma aberration is produced, the spot of laser light shone on the optical disc deviates from the proper position, and, when the coma aberration becomes larger than permitted, inconveniently, it becomes impossible to accurately write or read information. As a means to correct wavefront aberrations such as the coma aberration, there have conventionally been proposed some methods for correcting aberrations by the use of a variable-shape mirror.
For example, JP-A-H05-333274 proposes a method of performing phase control by varying the shape of the mirror itself of a variable-shape mirror by the use of a plurality of actuators. Disadvantageously, however, this method is unsuitable for use in a small component such as an optical pickup device because it does not take wiring and other factors into consideration. Moreover, it is difficult, both technically and in terms of cost, to miniaturize multi-layered piezoelectric elements used as those actuators.
JP-A-2004-109562 proposes that it is advantageous in terms of low voltage and miniaturization to correct wavefront aberration with a wavefront aberration correction mirror having a unimorph or bimorph shape provided with a piezoelectric element. It is also proposed that a mirror and the piezoelectric element are bonded together with adhesive in the variable-shape mirror constructed in this way.
However, when an optical pickup device provided with a variable-shape mirror 101 that varies a mirror surface by exploiting a lateral displacement of a piezoelectric element as shown in FIG. 4 is used to correct wavefront aberration, the following problems will arise. Here, FIG. 4 is an exploded perspective view showing components constituting the variable-shape mirror 101.
When piezoelectric elements 104 formed of PZT (lead zirconate titanate, Pb(ZrxTi1-x)O3) are bonded to both a mirror portion 103 formed of silicon and a support base 102 formed of glass, heat bonding will result in an insufficient bonding strength in the bonding portions due to different physical properties. In this case, when a mirror surface of the mirror portion 103 is varied by driving the piezoelectric elements 104 as shown in FIG. 5, bonding portions 105a between the piezoelectric elements 104 and the mirror portion 103, and bonding portions 105b between the piezoelectric elements 104 and the support base 102 are placed under load. Here, FIG. 5 is a sectional view as cut along line b-b shown in FIG. 4, showing a state in which the left-hand and right-hand piezoelectric elements 104 expand.
This increases, when bonding strength in the bonding portions is insufficient, the possibility that the bonding between the piezoelectric elements 104 and the mirror portion 103 or the bonding between the piezoelectric elements 104 and the support base 102 is broken when the piezoelectric element 104 is driven, reducing the reliability of the variable-shape mirror in terms of mechanical strength. Moreover, as the amount of expansion and contraction of the piezoelectric elements 4 is increased, the load placed on the bonding portions 105a and 105b is increased. This makes it impossible to increase the amount of expansion and contraction of the piezoelectric elements 104, thereby narrowing the aberration correction range.
For this reason, as proposed in JP-A-2004-109562, an adhesive can be used between the mirror portion 103 and the piezoelectric elements 104 to increase the bonding strength between them. In this case, however, the following problem will arise. When the mirror portion 103 is formed of silicon, its electrical conductivity eliminates the need to expressly form an electrode. However, when an adhesive is used between the mirror portion 103 and the piezoelectric elements 104, electrical conduction between the mirror portion 103 and the piezoelectric elements 104 is blocked. This makes it necessary to expressly form an electrode portion. That is, the use of an adhesive complicates the construction of the variable-shape mirror, and thus inconveniently increases the number of components.